There are two major lines of lymphoid cells for distinct immunological functions: the thymus-derived T cells for cell-mediated immunity and the bursa or bone marrow-derived B cells for humoral immunity, production of antibodies. Separation of these two cell lines can be easily accomplished in the chicken by in ovo bursectomy. The lymphoid tissues of bursectomized chickens are made up of only T cells and do not produce antibodies. Using these chickens, we have studied the antigen-binding properties of T cells. Chicken T cells are capable of binding soluble protein antigen in the absence of macrophages. Antigen binding to T cells occurs more efficiently at 37 degrees C than at 4 degrees C. The number of specific antigen-binding cells increases after immunization. These properties are characteristics of chicken T cells and are similar to those of mouse T cells. Antigen-binding T cells could be selected and enriched by adherence to antigen covalently linked to a solid support. Functionally, antigen-binding t cells mediate suppressor activity in antibody response to thymus-dependent antigens. Currently, we are trying to isolate and characterize surface membrane proteins that bind antigen. The chicken is ideal for isolating T-cell receptors because this animal provides a large number of antigen-specific T cells compared to much smaller animals such as mice or guinea pigs where surgical dissection equivalent to bursectomy is impossible.